Silicon carbide (i.e., SiC) is attracting attention as material of a power device since the silicon carbide has high electric breakdown strength. Since the SiC has high breakdown strength, the power device formed from SiC can control large current. Accordingly, application of SiC for a hybrid vehicle is expected.
To flow large current in a power device, it is effective to increase a channel density. In a silicon transistor, a vertical type trench gate power MOSFET is used for the power device. This structure is also applied to a SiC transistor.
When the SiC is used for the power device, a face orientation of a sidewall of a trench for defining a channel region is set to be a certain face orientation, which provides high channel mobility since the channel mobility largely depends on the face orientation. This is disclosed in JP-A-2007-80971 corresponding to US 2007/0057262.
In the vertical type trench gate power MOSFET, a P type base region for providing a channel region is formed, and a N+ type source region is formed on the base region. The impurity concentration of the source region is high so that the source region contacts a source electrode with ohmic contact. A trench is formed to penetrate the source region and the base region. The trench provides a trench gate structure. A corner of the trench is rounded so that an electric field concentration is reduced. Specifically, the trench is etched by hydrogen so as to round the corner of the trench.
However, when the corner of the trench is rounded, the source region is side-etched. When a side-etching amount of the source region is large, not only the source region but also the base region is side-etched, so that the base region is also rounded. Thus, the sidewall of the trench for defining the channel region may include a part having a face orientation different from the certain face orientation. In this case, since the part of the sidewall has the face orientation, which does not provide high channel mobility, the required channel mobility is not obtained. The inventors have preliminary studied about a relationship between the channel mobility and side-etching effect. FIG. 5 shows the relationship of the channel mobility. When the side-etching of the base region is not performed so that the trench does not include the part having the face orientation different from the certain face orientation, a whole of the channel region is provided by the certain face orientation. When the side-etching of the base region is performed so that the trench includes the part having the face orientation different from the certain face orientation, a part of the channel region is not provided by the certain face orientation. The channel mobility in a case where the side-etching of the base region is not performed is larger than that in a case where the side-etching of the base region is performed. Thus, the channel mobility is largely reduced.
To improve the reduction of the channel mobility, the sidewall of the trench is controlled to have the certain face orientation precisely. Thus, a process window for the hydrogen etching process and source region forming process becomes narrow. Thus, it is difficult to modify in-pane variation, which is generated necessarily in a forming process.